Indian scientists measure Van der Waals force using lasers

New Delhi: Scientists at the Indian Institute of Science Education and Research (IISER), Kolkata, have measured the Van der Waals force - that allows geckos to walk effortlessly along walls, and rainwater to accumulate and fall from a canopy - using optical tweezers.

The Van der Waals (VdW) force - named after Dutch scientist Johannes Diderik van der Waals - arises when two surfaces are brought into close proximity of each other, and the IISER Kolkata researchers measured it using oscillation optical tweezers constructed using the focused light from lasers.

VdW is of ubiquitous nature with its manifestations found all around us - starting from how a gecko sticks to a wall to how rainwater accumulated on a canopy falls down in the form of droplets.

However, the force acts over a very short spatial range, and dies off when the inter-surface separation is increased to even a few hundreds of nanometres. Measuring this force is very challenging - both because it is very small, and because its influence falls off rapidly with distance, Ayan Banerjee, from the Department of Physical Sciences, IISER Kolkata, told PTI.

In the peer-reviewed journal Applied Physics Letters, researchers noted that optical tweezers - with its capacity to measure forces as small as tens of femto-Newtons (10^-15 N) - can be a suitable candidate to measure such forces.

"This is what we demonstrate in this paper, where we use an optically trapped probe particle of three microns diameter to come up to 80 nanometres of a second larger particle, and measure how the VdW force influences the motion of the probe," Banerjee said.

Van der Waals force plays a fundamental role in fields as diverse as molecular chemistry, structural biology, polymer science, nanotechnology, surface science, and condensed matter physics. VdW decreases rapidly as the surfaces move away from each other, and is only very large when the surfaces are almost touching.

However, for tiny objects such as cells, where the forces concerned are themselves very small, the VdW force is of consequence even when the surfaces concerned are hundreds of nanometres away. This actually is not too small a distance with reference to a cell, where the DNA can only be tens of nanometres long.

Thus, surface forces are of very significant importance in the case of cells since they regulate cell adhesion, and phagocytic engulfment - a process where a cell surrounds and eats an invader.